Prolonging and deepening anaesthesia
General comments:
It is best to avoid having to prolong or deepen an anaesthetic in field conditions, and this is generally achieved by using high enough anaesthetic doses to induce anaesthesia effectively in the first place and working efficiently to limit the time an animal needs to be anaesthetised. However, there will be occasions when an induction is barely adequate for safety reasons, or when a little extra immobilization time is required to complete sample collection, collaring, etc. NB: The risk of anaesthesia is related to the length of the anaesthetic. In other words, short anaesthetics tend to be safer than long ones.
Injectable anaesthetic techniques:
Anaesthesia can be deepened and prolonged with incremental intravenous doses of ketamine every 20 minutes or so. The drug should be given slowly to effect in small doses of between 0.5 and 1 mg/kg. Although such an approach will inevitably increase recovery times, it will do so to a far lesser extent than with the intermittent use of intramuscular ketamine. Furthermore, the response to additional ketamine is far more immediate if given intravenously.
When anaesthesia has been induced with “Zoletil” or mixtures of drugs including “Zoletil”, do not use further “Zoletil” to deepen or prolong unless necessary due to the unavailability of ketamine. When using intravenous ketamine to deepen anaesthesia following inadequate induction with a medetomidine-ketamine mixture, give 25% of the dose of ketamine in the induction mixture. Medetomidine enhances the effects of most anaesthetics so incremental doses of ketamine should be kept low.
Intravenous ketamine may cause convulsions (seizures) which can be controlled with midazolam or diazepam if severe. See Anaesthetic emergencies – Seizures
Inhaled anaesthetic techniques:
Unlike injectable anaesthetics gaseous anaesthesia using volatile inhalant anaesthetic agents such as isoflurane or sevoflurane in a carrier gas (usually oxygen) is administered and eliminated via the respiratory system. By this means anaesthetic depth can be adjusted and recoveries kept short. However, compressed oxygen is generally required and special equipment needed to vaporise the anaesthetic agents and deliver the resultant gas to the patient via an endotracheal tube or face mask. Volatile agent anaesthesia is not without risk, and should only be used by suitably trained and experienced staff. Isoflurane and sevoflurane are potent cardiovascular and respiratory depressants and in inexperienced hands overdoses are all too easy to give.
Simple, portable field gas anaesthetic machines using compressed oxygen as the carrier gas can be custom built or obtained commercially (e.g. the “Stinger BackPak anaesthesia system” made by Advanced Anaesthesia Specialists) although their use is limited by the amount of oxygen that is practical to carry into the field. A very informative and comprehensive review of the subject can be found in Chinnadurai, 2019 (see Field anaesthesia references). Other portable anaesthetic systems overcome this limitation by using atmospheric air as the carrier gas (eg: isoflurane-air equipment, Lewis, 2004), but these systems cannot provide supplementary oxygen to a hypoxic animal.
Isoflurane (or sevoflurane) may not vaporize sufficiently at low temperatures, so it is important to keep the anaesthetic equipment warm when ambient temperatures are low. This can be done using an insulated box or bag containing disposable chemical hand-warmers or a lighter fuel powered hand warmer.
COMPREHENSIVE THEORETICAL AND PRACTICAL TRAINING IN THE USE OF GAS ANAESTHETIC EQUIPMENT IS STONGLY ADVISED BEFORE USE IN THE FIELD.